Recently, with the development of electrode materials and electrode structures having high power durability, surface acoustic wave filters have become necessary for duplexers that serve to separate transmission signals from reception signals in mobile communication devices.
The duplexers need to separate signals of a transmission band sufficiently from signals of a reception band close to the signals of the transmission band. For example, if a transmission band is at frequencies lower than a reception band, the duplexers need to have steep shoulder characteristics at a high frequency region of the transmission band. A circuit structure with a ladder filter with excellent power durability is generally used in a transmission filter passing signals amplified by a power amplifier.
Further, a technique for adding a capacitance in parallel with a resonator of a ladder filter is known to obtain steep shoulder characteristics of the ladder filter.
The recent trend toward modification of the communication standards and global roaming of mobile phones has prompted the requirement to attenuate strictly the frequency bands that are used as another standard near the passbands of duplexers.